The present invention relates to imaging devices, and in particular, to a method for compensating non-uniformity of imaging device response.
Image sensors are widely used in devices such as camcorders and digital cameras. Generally, an image sensor comprises a pixel array where each pixel has a photosensitive area. According to one type of image sensor, the pixels have photodiodes that operate according to photoelectric principles. Stated differently, each photodiode captures a portion of the instant light that falls on the pixel array and converts it into an electrical signal. The electrical signals converted by the photodiodes then can be used to recreate the scene captured in the instant light.
To further aid in the understanding of the invention, a complimentary metal oxide semi-conductor (CMOS) image sensor will be used as an example. FIG. 1 illustrates a pixel of the CMOS sensor. The pixel 10 of the CMOS sensor is formed on a silicon substrate 12 and comprises a photosensitive area 14 such as a photodiode. The pixel may be formed using photo-lithographic techniques. A color filter 16 is placed on top of the photosensitive area 14 that allows one primary color (e.g., red, green or blue) to pass through to the photosensitive area 14. The color filter 16 may be applied to the pixel using existing commercial color filter array (CFA) materials. To increase the photosensitivity of the photosensitive area 14, a micro-lens 18 may be formed over the photosensitive area 14 and the color filter 16. The pixel 10 further comprises other semiconductor devices such as transistors which process the electrical signal generated by the photosensitive area. Therefore, generally, the photosensitive area occupies a portion the overall pixel area.
One of the problems associated with an image sensor is that because the photosensitive area occupies a portion of the pixel, each pixel has an acceptance angle in which the photosensitive area is responsive to the instant light falling on the pixel. Stated differently, only instant light that falls up to a certain angle normal to the surface of the pixel will be detected by the light sensitive area of the pixel. Another problem associated with the image sensor is that the lens covering the pixel does not uniformly pass the instant light to the light sensitive area of the pixel. Stated differently, deformities and impurities in the lens absorb a portion of the light that passes through it at different levels for the various lenses on the various pixels.
The mentioned problems, including others, cause the image sensor to have a response that is not the same for all pixels even when a uniform illuminating light has been applied to the image sensor. For example, under a uniform illumination, due to the limitations described above, the readouts obtained from the pixels around the image sensor center is higher than the readouts near the image sensor periphery. Such phenomena is undesirable in that the captured images would have a non-uniform appearance, even though such is not the case in the original scene. The phenomena is more noticeable in the case where the scene has a uniform or simple background. Therefore, it is desirable to alleviate this perceptual non-uniformity and to improve the overall image quality.
A method and apparatus for compensating non-uniformity of imaging device response is described. A plurality of locations on the imaging device is selected. The light intensity for each of the selected locations is measured. The measured light intensities are normalized using a reference light intensity and the normalized light intensities are stored in a memory.
Additional features, embodiments, and benefits will be evident in view of the figures and detailed description presented herein.